1. Technical Field
This invention relates to a submicron bipolar transistor and, more particularly, to such a device formed with horizontal conducting layers between the sidewall edges of the active regions and the metal contacting electrodes.
2. Description of the Prior Art
The direction of technology in the field of semiconductor devices is toward a device structure with high speed and low power consumption. This structure demands active regions which are shallow in depth and placed as close together as possible.
One exemplary process for fabricating relatively small bipolar devices is disclosed in U.S. Pat. No. 4,481,706 issued to M. Roche on Nov. 13, 1894. Roche utilizes vertical layers of oxide and nitride on the sidewalls of the region where a base implant has been made to define the outer boundaries of the emitter region. The thickness of the oxide and nitride layers thus results in providing an extremely narrow emitter region. An alternative fabrication method is disclosed in U.S. Pat. No. 4,495,010 issued to D. Kranzer on Jan. 22, 1985 wherein ion implantation of an undoped polysilicon layer is used to form base and emitter regions which are very thin and also very small.
A problem with these and similar prior art configurations is the requirement of a buried collector region with a relatively large lateral extension so as to provide for the connection to a collector electrode on the top surface of the device. An additional implant between the device surface and this buried region (this implant is commonly referred to as a collector reach-through) is thus required to complete the connection between the collector diffusion and the surface electrode. This obviously results in the formation of a device which covers a larger substrate surface area than is actually required to perform the transistor function.
There exist lateral bipolar transistor structures which eliminate the need for a buried collector region by forming the collector, base, and emitter regions as adjacent diffusions at the surface of the substrate. Examples of this structure may be found in U.S. Pat. No. 4,641,170 issued to S. Ogura et al on Feb. 3, 1987. However, device size and speed limitations still remain with this lateral structure.
A size limitation that exists with virtually every bipolar device structure is the required separation between the electrical contacts to the device areas. Most notably, the conventional device structures described above which use an extensive lateral buried subcollector, resulting in the formation of a collector electrical contact far removed from the base and emitter electrodes.
In light of the above, there remains a need in the prior art for a bipolar device which is small in size and capable of achieving high speed operation, which requires neither the use of a large buried collector region nor the formation of a lateral transistor diffusion region.